At some point one is certainly entitled not to rehash old arguments. I do not personally get involved in discussions of the trinity as a doctrine; there's almost no chance that anyone will present me with an argument I haven't seen before.
I count this, however, as a personal prerogative, not some doctrinal statement about my authority.
So over in Ponty's blog we get into this long thread about Mariology, and in comment #30 he says:
"#23: Your presupposition is “If the popes and Eastern patriarchs have endorsed it, it is correct.” But of course, Protestantism is practically defined by its rejection of some sort of generative infallibility among a certain class of people.
"Exactly! And that is precisely why Protestantism is incapable of holding on to the fullness of the faith in its confrontation with modernity."
Well, in a sense Catholicism has been incapable of holding on to it too, for Catholicism spawned Protestantism. Claiming infallibility is fairly effective in cutting off interminable debate of basic principles, at least internally. It does nothing at all about cutting them off externally. And in fact, it presents a tradeoff. The ultimate defense of arguments is being tested against refutation; hence, anything that curtails those tests reduces confidence in those arguments.
In practice, cutting off debate is a temptation to sin by ignoring the refutation of poor arguments. In the world at large, debate cannot be cut off, but it can be crippled by poor communication. Hence bad RC and EO doctrinal claims are refuted every day; it's just that RC and EO authorities ignore these refutations. And I mean "refuted" in a very specific and objective sense: they are found to be not valid by individuals.
As soon as one has to talk about infallibility, one is admitting that human reasoning is not all that it could be-- and not because of the limitations of reason, but because sin contaminates reasoning. It's obvious that claims to infallibility occaisions of near (if not actual) sin, due to the temptation to dress one's assertions up in supernal authority. But such assertions should not be refutable even in ordinary reasoning.
Which brings me back to the original quote. Even if the "infallible" churches are less tempted to dump basic items of faith, I do not think they are protected from obscuring these truths in a thicket of errors or extraneous claims and practices. Mariology stands as an object case: one need not look far to see places in the RC church where devotion to Mary is elevated practically to worship, and Jesus becomes an appendage.
Also, infallibility must stand on its own feet. If it is needed to justify dogmatizing certain principles, the more fundamental solution is to dogmatize them directly. The more controversial propositions infallibility is stretched to cover, the more people are encouraged to doubt the fundamentals. In that wise, Protestantism is nothing more than the acknowledgement of the real state of human judgement.